(1) Field of the Invention
The present invention relates to variable resistors for mounting on a surface such as a printed circuit board, which have resistance adjustable by rotating a slider on a resistive element formed on an insulating substrate.
(2) Description of the Prior Art
A known example of such variable resistor is illustrated in FIGS. 1 and 2A through 2C. FIG. 1 is an exploded perspective view of the variable resistor, FIG. 2A is a plan view, FIG. 2B is a section taken on line A--A of FIG. 2A, and FIG. 2C is a bottom view of the variable resistor.
The illustrated variable resistor comprises an insulating substrate 1 defining a bore 1a substantially centrally thereof. The substrate 1 includes an arcuate resistive element 2 formed on a surface thereof concentrically with the bore 1a, and external electrodes 3 and 4 formed, for instance, by printing and baking silver paste onto the insulating substrate 1 to extend from the surface to an end face and the bottom surface, respectively. One of the electrodes 3 is electrically connected to one end of the resistive element 2 and the other electrode 4 to the other end thereof. Number 5 indicates an electrode formed of a metallic plate and including a hollow center cylinder 6 formed integrally with the electrode 5 such as by the drawing process. The electrode 5 is fixed to the bottom surface and an end face of the insulating substrate 1 with the center cylinder 6 inserted into the bore 1a of the substrate 1. Number 7 indicates a slider defining a bore 7a substantially centrally thereof, a contact 7b peripherally thereof for contacting the resistive element 2, and a U-shaped cutout 7c. Number 8 indicates a metallic rotor defining a bore 8a substantially centrally thereof, driver grooves 8b in an upper surface, and a projection 8c at one peripheral position on its bottom surface. The center cylinder 6 of the electrode 5 extends through the bores 7a and 8a of the slider 7 and rotor 8, with the projection 8c engaging the U-shaped cutout 7c of the slider 7. The slider 7 and rotor 8 are rotatably assembled with the insulating substrate 1 by calking the distal end of the cylinder 6. It will be appreciated that the slider 7 and rotor 8 are rotatable in unison by the engagement between the cutout 7c and projection 8c. According to this variable resistor, the slider 7 and rotor 8 are rotatable by fitting the tip of a driver in the driver grooves 8b. The rotation causes the contact 7b to slide on the resistive element 2, thereby to adjust the resistance between the external electrode 3 or 4 and the electrode 5.
Another known variable resistor is illustrated in FIGS. 3A and 3B. This variable resistor is contained in a case 13 and is adapted for mounting on a surface by flow soldering. In FIGS. 4A and 3B like parts of the variable resistor are labeled with like reference numerals with respect to the variable resistor shown in FIGS. 1 and 2A through 2C and will not be described again.
This variable resistor includes electrodes 9, 10 and 11 formed of metallic plates. The electrodes 9 and 10 are attached to one end face of an insulating substrate 1 and electrically connected to opposite ends of a resistive element 2, respectively. These electrodes 9 and 10 correspond to the electrodes 3 and 4 of the foregoing variable resistor, and the electrode 11 to the electrode 5 thereof. The electrode 11 is electrically connected to a center rod 12 corresponding the center cylinder 6 of the foregoing variable resistor. The slider 7 and rotor 8 are rotatably mounted on the insulating substrate 1 by calking a distal end of the center rod 12 in the bore 8a of the rotor 8. Number 13 indicates a plastic case having a top opening. The insulating substrate 1 is insert molded with the electrode 9, 10 and 11 extending outwardly of the case 13 and a surface of the substrate 1 exposed to a bottom inside surface of the case 13. The outwardly extending electrodes 9, 10 and 11 are bent over to the bottom of the case 13. Number 14 indicates a heat-resistant film covering secured peripherally of the top opening of the case 13 to seal the opening. The film covering 14 is formed of a transparent material through which the interior parts of the variable resistor may be seen as depicted in FIG. 3A.
This variable resistor is mounted on a printed circuit board or the like such as by flow soldering, and thereafter the film covering may be broken with a driver or the like. Then the slider 7 and rotor 8 are turned with the driver to cause a contact 7b to slide on a resistive element 2 for adjusting the resistance.
The above two known variable resistors have the following disadvantages.
The variable resistor shown in FIGS. 1 and 2, because the resistive element 2 and slider 7 are exposed outside, cannot be mounted on a printed circuit board or the like by flow soldering which provides high productivity. Thus, the variable resistor must depend on reflow soldering for mounting on the circuit board or the like. However, even in mounting by reflow soldering, flux tends to be scattered onto the resistive element 2 and slider 7 to cause imperfect contact.
The variable resistor shown in FIG. 3 which is contained in the case 13 can be mounted on a circuit board by flow soldering, but includes many parts and, therefore, is troublesome and costly to manufacture. Moreover, when breaking the film covering 14 with a driver or the like, fragments of the film covering 14 tend to be scattered inside and outside the case 13 thereby causing imperfect contact for the variable resistor itself or producing a detrimental effect on the device on which the variable resistor is mounted. There is a further disadvantage in that, once the film covering 14 is broken, the device on which the variable resistor is mounted becomes no longer adjustable or washable because the resistive element 2 and slider 7 are exposed to the outside.